As disclosed in our co-pending applications Ser. Nos. 08/719,385 and 08/880,990, herein incorporated by reference, the art is replete with references to palladium-bearing hydrogen selective membranes and to their use in hydrogen purifiers and reactors and the like. We have stressed in said applications the advantages of palladium alloy membranes in the form of sheet membranes, which have been made significantly thinner than the walls of the commonly used small-diameter tubular membranes. The result has been improved hydrogen permeation as well as significant palladium savings.
As used at times in this specification and the appended claims, the term "hydrogen selective palladium-bearing metal" is intended to embrace palladium metal or a hydrogen selective palladium alloy, and the term "sheet membrane" means a sheet obtained by cold-rolling a palladium-bearing metal billet to a thickness sufficient to insure substantial freedom of pinholes.
The cost and difficulty of the before-mentioned cold-rolling of billets to substantially pinhole-free sheets, however, increases sharply as the sheet thickness decreases. In practice, substantially pin hole-free, free standing sheets of two typical hydrogen-permeable alloys consisting of palladium and copper, about 60% and about 40% (plus or minus ca.3%), respectively, by weight, (herein "Pd60/Cu40") and palladium and silver, similarly about 75% and about 25%, respectively, by weight, (herein "Pd75/Ag25") are not available commercially below a thickness of about 0.001 inch.
The method of fabrication underlying the present invention overcomes the practical limitations in thinness of such cold-rolling techniques and now enables thinner sheet membranes to be practically obtained.
The desirability of this advance is evident from the consideration that the permeability of hydrogen through a free-standing palladium-bearing membrane increases about linearly with decreasing thickness, and this, at the same time, results especially in decreasing the membrane palladium metal content. As the membranes need to be incorporated in purifier and/or reactor equipment and the like, the double advantage of a higher permeation flux with a lower palladium content is further magnified in the resulting lowered equipment cost.